1. Field of the Invention
The present invention relates to an optical pulse tester (hereinafter referred to as an OTDR) for changing the gain of a measured circuit according to the S/N ratio of measurement results in order to obtain a measurement waveform having a desired S/N ratio.
2. Background Art
An OTDR sends an optical pulse through an optical directional coupler into an optical fiber being measured, then detects flaws and measures loss in the fiber or connection loss by detecting the returned optical pulse.
Next, the structure of a conventional OTDR shall be explained with reference to FIG. 7.
In FIG. 7, reference numeral 1 denotes a timing generating section, reference numeral 2 denotes a drive circuit, reference numeral 3 denotes a light source such as a laser, reference numeral 4 denotes an optical directional coupler, reference numeral 5 denotes an optical receiver, reference numeral 6 denotes an amplifying section, reference numeral 7 denotes a digital processing section, reference numeral 8 denotes a display section, and reference numeral denotes a measured optical fiber which is the object of measurement.
With reference to FIG. 7, the drive circuit 2 releases a pulse current based on an electrical pulse from the timing generating section 1, to generate light from the light source 3. The optical pulse outputted from the light source 3 passes through the optical directional coupler and is incident on the measured optical fiber 10. The light returned from the measured optical fiber 10 such as back-scattered light or reflected light is sent from the optical directional coupler 4 to the optical receiver 5. The optical receiver 5 converts the optical signal into an electrical signal. The output of the optical receiver 5 is amplified by the amplifying section 6 and converted into a digital signal by the digital processing section 7, then is processed for noise reduction such as by averaging. Furthermore, the results have been logarithmically converted, then displayed on the display section 8.
In FIG. 7, the back-scattered light which returns from the measured optical fiber 10 is caused by Rayleigh scattering occurring within the measured optical fiber 10. The intensity level of this back-scattered light is lower than the incident optical pulse intensity by approximately 50 dBs, assuming that the measured optical fiber 10 is a normal single-mode fiber and the optical pulse width of the incident light is 1.times.10.sup.-6 seconds long. In order to handle such a weak signal, the apparatus shown in FIG. 7 repeatedly measures the back-scattered light and takes the average value in order to improve the S/N ratio. The digital processing section 7 is provided for this reason.
Next, a measured waveform from a conventional OTDR shall be explained with reference to FIG. 8. FIG. 8 shows a case wherein two different optical fibers are connected and these are measured as the measured optical fiber 10. The horizontal axis denotes the optical fiber length (the time after the optical pulse has been emitted by the light source 3 and directed to the measured optical fiber 10), and the vertical axis denotes the received optical intensity level.
As shown in FIG. 8, the measured waveform exhibits a straight line which decreases toward the right due to logarithmic conversion by the digital processing section 7. Additionally, the loss at the junction portion of the two optical fibers is shown as a downward step, and reflection at the connectors and reflection at the terminal points are observed as a large discontinuous upward wave.
The maximum value of the window shown in FIG. 8 is the maximum value inputted into the digital processing section 7 at a certain gain designated by the amplifying section 6, and the minimum value of the window shown in FIG. 8 is the value of noise at the input end of the digital processing section 7 due to the light received from the measured optical fiber 10 being so weak as to be lost in the noise of the optical receiver 5. As shown in this drawing, in a conventional OTDR, when the received light level is high and close to the maximum value, the S/N ratio is good and a clean waveform is observed; conversely, when the received light level is low, the S/N ratio is poor and noise is superimposed onto the observed waveform so as to result in a widely spread waveform. Additionally, with regard to FIG. 8, the width of the window of observation (maximum value to minimum value) may be considered to be the measurement range of common circuit testers. For this reason, when the received light level exceeds the maximum value of the window and the measurable range is exceeded, then the tester is set to a higher range, and when the received light level is low and the measured value is extremely small with respect to the range, then the tester is set to a lower range in order to take measurements. In this case, when the level changes greatly on the time axis as with the received waveform of an OTDR, the received waveform only rarely fits inside the window in a single measurement. Therefore, the measurements must be taken while switching the measurement range many times.
The applicant has filed a patent application for an OTDR which resolves these problems (see Japanese Patent Application, First Publication No. Hei 4-158237). Hereinbelow, this OTDR shall be described briefly.
The OTDR has a pre-designated S/N ratio, and comprises S/N ratio comparing means for comparing the received light level which has been logarithmically converted (corresponding to the output of the digital processing section 7 in FIG. 7) with the pre-designated value, and data storage means for storing the received light level. The received light levels at the portions where the logarithmically converted received light levels are better than the pre-designated value (pre-designated S/N ratio) are stored in the data storage means. With regard to the portions at which the received light levels are worse than the pre-designated value, comparison and storage are repeated while improving the S/N ratio by increasing the gain of the amplifying portion, or improving the S/N ratio by increasing the number of times the back-scattered light is added in order to find the mean value. Finally, the data stored in the data storage means are joined together and displayed.
However, the above-described OTDR (described in Japanese Patent Application, First Publication No. Hei 4-158237) leaves a problem in that when the data is ultimately joined together, the received light levels of the data are difficult to join together because the standards by which the received light levels of the data are to be joined together are not clear from only the contents stored in the data storage means.
That is, since the data storage means stores only the received light levels at each distance, when joining together data with the above-described OTDR, the mutual received light levels must be adjusted such that the received light levels are matched and vary smoothly at the joint portions of the waveforms between adjacent data areas. Consequently, a problem remains in that when adjacent data does not exist, the received light levels of the data are not capable of being decided.